The disclosure of U.S. Pat. No. 5,130,005 entitled MAGNETRON SPUTTER COATING METHOD AND APPARATUS WITH ROTATING MAGNET CATHODE filed Dec. 13, 1990 is hereby incorporated by reference.
The present invention relates to a cathode used in a sputtering process, and more particularly, to a sputtering cathode assembly with a variable magnetic configuration that compensates for variations in deposition film thickness caused by erosion of a sputtering target.
In a sputtering process, sputtering cathode assemblies are commonly used to deposit metal or other conductive material as a thin film onto a surface of a substrate such as a semiconductor wafer. Sputtering cathode assemblies include a vacuum chamber in which a sputtering target is positioned in close proximity to the substrate. As energy is applied to the target, atoms are ultimately sputtered or dislodged from the surface of the target and deposited on substrate.
Frequently, non-uniformities in a film are a result of there being more of a target structure (i.e. a sputtering source) located near a center of a substrate than at an outer edge of the substrate. In theory, with all other parameters being equal, an infinitely large target with a uniform erosion profile would be capable of overcoming film non-uniformity due to the target size. Therefore, it is desirable to make the target large with respect to the surface of the substrate being coated. There are, however, practical limitations on the size of the sputtering target which may be used. Usually, a compromise is made by providing a target having a diameter that is approximately one-half larger than that of the substrate being coated. Nonetheless, with this compromise, use of a target which is eroded uniformly still results in the formation of a film whose thickness increases when moving from the edge of the substrate toward the center of the substrate.
In many commercial processes, such as those where a high rate of deposition is a consideration, magnetron enhanced plasma generation and shaping techniques are utilized to establish a desired erosion profile of the sputtering target. More recently, sputter coating equipment manufacturers have provided sputter coating target and cathode assemblies in which rotating magnet packages are used to assist in magnetron enhancement of the plasma. One such rotating magnet cathode assembly is described and illustrated in U.S. Pat. No. 5,130,005 entitled MAGNETRON SPUTTER COATING METHOD AND APPARATUS WITH ROTATING MAGNET CATHODE which is assigned to Materials Research Corporation.
Designs for cathode assemblies have been optimized to shape the erosion profiles of the targets to compensate for the thinner edge coating effect and other factors contributing to non-uniformity of the film. This is frequently achieved by selectively configuring the magnet assemblies used to shape the plasma. For example, in order to compensate for thinness of a film at the substrate edge, magnet assemblies are often designed to produce a greater duration of plasma around the target edge. This causes the target to erode at a higher rate near the target edge to compensate for reduced target exposure near the substrate edge.
Referring to FIGS. 1 and 2, a rotating magnet assembly is shown. A magnet carrier plate 20 is arranged to be rotatable around center axis 26 on shaft 18. The plate 20 includes an array of magnetic material 28 having a fixed predetermined shape. In particular, the magnetic material 28 is selectively configured so that there are portions positioned at various radii from the center axis 26 in an irregular partially-formed loop. In this configuration, magnetic material 28 is oriented such that a north magnetic pole is at an outer perimeter face 32 while a south magnetic pole is at inner perimeter face 34. The shape of the sputtering plasma and the resultant erosion of first target 72 adjacent magnetic material 28 is defined by a magnetic flux path 76 extending between the north 32 and south 34 magnetic poles which create a closed loop magnetic field tunnel on the face of the first target 72.
When the magnetic material 28 is rotated with respect to first target 72, an erosion profile 74 is formed in first target 72. The profile 74 includes a substantially circular xe2x80x9cbulls eyexe2x80x9d pattern wherein the deeper and shallower portions of profile 74 are formed by the particular pattern shape of magnetic material 28. A first outermost portion 78 of profile 74 is dominated by a rotational dwell of outermost areas 79 of the magnetic material 28, while the depth of erosion of a central portion 82 of the first target 72 is dominated by center areas 84 of magnetic material 28. Similarly, an erosion of an intermediate portion 86 between the first outermost portion 78 and the center portion 82 results from a rotational dwell of intermediate areas 88 of magnetic material 28.
The shape of the profile 74, in addition to other factors such as the size of the first target 72 and substrate 92 and the spacing between first target 72 and substrate 92 are some factors which determine film thickness uniformity. Further, thickness uniformity is not constant during the life of the target. When first target 72 is new, an initial sputtering surface 90 is substantially flat and at a known distance from the substrate 92 which is to be coated with sputtered material. As erosion progresses, the surface 90 gradually assumes the shape of profile 74. Together with the erosion, the distance between the surface 90 and substrate 92 increases.
The effect of these changes is shown in FIG. 3. FIG. 3 illustrates sputtered material thickness across a diameter of a six-inch substrate. Curve 94 illustrates sputtered material thickness obtained when first target 72 is new and surface 90 is substantially flat. As target erosion progresses, uniformity degrades as illustrated by curve 96 at the mid-point of target life. As first target 72 approaches the end of life, the thickness of sputtered material degrades to that illustrated by curve 98. The progression of sputtered material thickness as first target 72 erodes, indicates a reduction of film thickness towards the edge of substrate 92.
These deficiencies may be corrected by periodically replacing the magnetic material 28 with magnetic material having a different shape adapted to provide a higher dwell time in the outermost areas 79, thereby causing increased sputtering in the first outermost portion 78 of the first target 72. However, replacement of magnetic material 28 requires removal of the plate 20 and shutdown of sputtering system, resulting in increased costs and reduced productivity.
Further, it is known that sputtering targets of different materials or crystal structures perform differently in the sputtering process. Referring to FIG. 4A, an angular distribution of material 120 sputtered from a location 122 on a second target 124 fabricated from aluminum is illustrated. This distribution shows that a majority of material sputtered is located predominantly normal to the target surface. Referring to FIG. 4B, an angular distribution of material 132 sputtered from a location 126 on a third target 128 fabricated from terbium or gold is shown. Here the sputtered material is distributed at lower angles along lobes 130 than that shown in FIG. 4A. Optimizing the deposition uniformity for these and other materials requires replacement of the magnet structure. However, as previously described, this requires undesirable shutdown of the sputtering system.
A sputtering system for depositing a thin film onto a substrate is disclosed wherein the system includes an evacuatable chamber which includes the substrate. In particular, the system includes a target positioned within the chamber, wherein the target includes a back surface and a sputtering surface. Further, the system includes plasma for eroding the target to provide material for forming the thin film wherein erosion of the target occurs in a predetermined erosion pattern and is controlled by a shape of the plasma. In addition, a magnet arrangement is provided which provides a magnetic field on the target for controlling the shape of the plasma, wherein the magnet arrangement is positioned adjacent the back surface. The system also includes a support for supporting the substrate opposite the sputtering surface and an adjustment arrangement for adjusting a dwell time of the magnetic field over predetermined portions of the target to change the shape of the plasma to thereby change the erosion pattern of the target.